The present invention relates to an ion trap mass spectrometer and ion trap mass analysis method.
A mass analysis apparatus is an apparatus for separating an ionized sample by controlling an electric field or a magnetic field. Nowadays, a mass filter type mass analysis apparatus and a magnetic field type mass analysis apparatus are generally used. These types of mass analysis apparatus ionize the sample continually, and scan the ionized sample passing through the electric field or the magnetic field so as to be mass-separated. On the other hand, another type of mass analysis apparatus was suggested by Paul et al. In this apparatus, the ions are closed in a trap space by a quadrupole radio frequency electric field, and after first accumulating the ions, they are mass-separated. For example, such a technology is known by U.S. Pat. No. 2,939,952 or U.S. Pat. No. 4,540,884, and International Journal of Mass Spectrometry and Ion Processes, 60(1984), Pages 85-98, xe2x80x9cRECENT IMPROVEMENTS IN AN ANALYTICAL APPLICATIONS OF ION TRAP TECHNOLOGYxe2x80x9d.
According to the above technology shown in the aforementioned U.S. Patents, after once accumulating the ions, when the ions are mass-analyzed, it becomes possible to mass-analyze them with a high sensitivity. However, as the ions are accumulated, problems are also inherent. That is, as the ions are accumulated over a predetermined time, the time that the ions are confined to the trap domain becomes longer. In the meantime, the ions forming the sample will have reacted with the active molecules etc. in the trap space. Thereby, the sample ions change to other ions, or become extinct, and a reduction in the mass analysis accuracy occurs.
An object of the present invention is to reduce with effect of the active gas in the trap space, thereby improving the accuracy of the mass analysis apparatus.
In order to achieve the above object, in the present invention, a trap space surrounded by electrodes is formed, ions are captured in the trap space by applying an alternating voltage to the electrodes, active molecules in the trap space are removed, and the ions which are captured are mass-separated after removing the active molecules.
In this way, where the active molecules remaining in the trap space are removed before mass-separating the sample ions, the effect of the active molecules in the trap space is reduced. Thereby the mass analysis accuracy is improved.
As the mass spectrum is obtained by the ion trap mass spectrometer after having once accumulated the introduced ions into the electrode used for the ion trapping a highly sensitive measurement becomes possible, and such highly sensitive measurement is performed by introducing an appropriate quantity of the buffer gas. If the gas pressure in the ion trap is to high or to low adversely, a good result is not obtained. As explained above, if the active molecules such as water or oxygen are mixed in the gas in the trap, the ions which are introduced enter into a mutual action with the active molecules, leading to adverse results such as loss of charge, transfer, ion molecular reaction etc., and become extinct or are changed to other ions which are different from the introduced ions.
This phenomena is conspicuously found in the first action of LC/MS apparatus. In order to keep the pressure in the ion trap constant, airtightness of the space between the three electrodes is raised by using a spacing piece such as ceramic or glass. In starting the evacuation, this airtightness obstructs exhaustion in the quadrupole. Even if the vacuum of circumference of the ion trap mass spectrometer is high enough for analysis, the vacuum level and partial pressure of the water in the ion trap electrodes are not at the level at which the analysis is fully performed. Therefore, in an early step when the evacuation starts, a big problem because a stable measurement is not possible. When the mass spectrometer stops, the inside of the electrodes is exposed to atmospheric air, and, as the polarity of the water and oxygen molecules are very high, they exist in a gaseous state and are absorbed on the inner wall surface of the quadrupole electrodes. These water molecules absorbed on the inner wall surface are removed progressively during the measurement, and enter into an ion molecular reaction with captured ions. This is a reason that the measurement becomes unstable for a long time. In order to prevent this problem, when the mass spectrometer is exposed to the atmosphere, the gas introduction from the buffer gas introduction system is continued after stopping the apparatus, and the enough inactive molecules are filled in the trap generally. After starting up the apparatus further, the exhaustion continues for more than overnight. However, thereby, the measurement starts after one day or more, and analysis efficiency falls remarkably.
Desirably in the present invention, in order to solve such problems stated above, the buffer gas is heated and is introduced in the quadrupole electrodes, thereby the water molecules absorbed to the inner wall of the electrode are removed. Moreover preferably, the quadrupole electrodes themselves are heated, and the removing time of the water molecules is shortened.
More preferably, before the measurement of the LC/MS, gas at high temperature is introduced so as to remove the water molecules, and after once starting the measurement, the temperature of the gas is controlled to be low and thermal degradation of the sample molecules is thereby prevented.
More preferably, the temperature setting of the trap electrode is raised while the measurement is not being performed and is controlled to be low just before the measurement starts. Thereby an even more stable measurement becomes possible.